Vision iOS App Altitude Accuracy?

I did an altitude test run last weekend and got to 2,503 feet as indicated on the IOS Vision app on my iPhone but the datalogger, a Flytrex, only reported 2,247 feet. I sent the info to Flytrex and they said the data from the the Phantom GPS matched the Flytrex within a couple of feet and that the 2,247 measurement was correct.

So, where is the Vision iOS app getting it's data? Anyone noticed this or have any input on reasons for the discrepancy?

The video of this is at: http://www.youtube.com/watch?v=p7Egt5_Rg-4, and I know...it's too long, I'll start editing these in the future. The top ascent is at about 4:10, the Vision App screen capture is at 13:00.

The P2V has both GPS and a barometric altimeter. Both are problematic; GPS altitude is inherently sloppy due to the poor geometry (you need the satellites blocked by the earth) and by the fact that its model doesn't exactly match the actual shape of the earth, and can jump around significantly while staying in one place. Barometric altimeters are more repeatable (going to the same place around the same time will give you roughly the same altitude) but are wildly inaccurate unless you adjust for the local barometric pressure (us full-scale guys set our altimeters to compensate). Baro altimeters are also nonlinear and get sloppier with altitude--their primary purpose is to keep airplanes from crashing into the ground (down low) and into each other (everywhere), so they don't have to be accurate in absolute terms once you get away from the surface.

From what I can tell, the Flytrex taps into the GPS feed from the DJI, so it is presumably reporting GPS altitude. There's no telling what the telemetry reports, but one possibility is that it calibrates the baro altimeter when the home point is set, based on the reported GPS altitude (giving an absolute reading), and then uses the baro altimeter to report the altitude as it flies.

The actual altitude is doubtlessly something other than either one reported, but presuming the telemetry feed is baro-based, it's probably more accurate, depending on the quality of the sensors of course.

Staff Member

The P2V has both GPS and a barometric altimeter. Both are problematic; GPS altitude is inherently sloppy due to the poor geometry (you need the satellites blocked by the earth) and by the fact that its model doesn't exactly match the actual shape of the earth, and can jump around significantly while staying in one place. Barometric altimeters are more repeatable (going to the same place around the same time will give you roughly the same altitude) but are wildly inaccurate unless you adjust for the local barometric pressure (us full-scale guys set our altimeters to compensate). Baro altimeters are also nonlinear and get sloppier with altitude--their primary purpose is to keep airplanes from crashing into the ground (down low) and into each other (everywhere), so they don't have to be accurate in absolute terms once you get away from the surface.

From what I can tell, the Flytrex taps into the GPS feed from the DJI, so it is presumably reporting GPS altitude. There's no telling what the telemetry reports, but one possibility is that it calibrates the baro altimeter when the home point is set, based on the reported GPS altitude (giving an absolute reading), and then uses the baro altimeter to report the altitude as it flies.

The actual altitude is doubtlessly something other than either one reported, but presuming the telemetry feed is baro-based, it's probably more accurate, depending on the quality of the sensors of course.

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Flytrex also has a baro. Depending on the age of the Flytrex, there was an issue with the foam packing around the baro which skewed the metrics. Flytrex instructs to, very carefully, remove that piece of foam. The newer models were slightly modified to eliminate this problem. When removing the foam you need to be extra careful not to score the top of the board, as that's where the circuitry is located...

We can tell from a pic of your Flytrex whether or not you have the older model...

Yes, I believe the Flytrex records altitude data using its own baro so it logs both altitude (above sea level) and max ascent (max height achieved above ground level).

What I don't know is what the app uses to get it's zero data. I suspect it might be using both the Vision baro and GPS in different circumstances, which might account for the variance in app reported data (and why some people have a ground reading that can be off by quite a few feet).

Staff Member

Yes, I believe the Flytrex records altitude data using its own baro so it logs both altitude (above sea level) and max ascent (max height achieved above ground level).

What I don't know is what the app uses to get it's zero data. I suspect it might be using both the Vision baro and GPS in different circumstances, which might account for the variance in app reported data (and why some people have a ground reading that can be off by quite a few feet).

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I either read exactly that in the documentation somewhere or it was on one of the vids. I'm pretty sure you're right that it uses both baro and GPS depending on circumstance. My ground reading is usually around 2-3 feet.

Yes, mine's been pretty good at reporting ground level as "zero" +/- a couple of feet. I'm not sure why it shouldn't be zero exactly at switch on - the app should just be saying "take whatever the current pressure reading is and make that zero feet, then for every 0.33 of a millibar the pressure drops increase the height by 1ft, cross-checking with GPS now and again in case you're close to a front where the ambient pressure is changing rapidly. DJI quotes the accuracy of it's height holding to about 2ft, which is definitely barometer-level accuracy, not GPS, so you should be, at most, +/- 2ft from it reading zero on the ground just after switch on...

Yes, mine's been pretty good at reporting ground level as "zero" +/- a couple of feet. I'm not sure why it shouldn't be zero exactly at switch on - the app should just be saying "take whatever the current pressure reading is and make that zero feet, then for every 0.33 of a millibar the pressure drops increase the height by 1ft, cross-checking with GPS now and again in case you're close to a front where the ambient pressure is changing rapidly. DJI quotes the accuracy of it's height holding to about 2ft, which is definitely barometer-level accuracy, not GPS, so you should be, at most, +/- 2ft from it reading zero on the ground just after switch on...

Maths was never my strong point. Although I'm still going to claim this was a typo (it was, I used a calculator to work it out, such is the paucity of my mental arithmetic ability, but I'm also a crap thumb typist ).

Staff Member

Maths was never my strong point. Although I'm still going to claim this was a typo (it was, I used a calculator to work it out, such is the paucity of my mental arithmetic ability, but I'm also a crap thumb typist ).

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I type at over 70 words per minute. But watching my Grand Daughter thumb type gives me a migraine. I'm all thumbs...

I've noticed a similar discrepancy between Flyrex and P2V's app. Apps said 4049' and Flytrex max was only 3853', about 5% less. I've not done much high altitude flying but I've never seen the error go the other way.

Staff Member

I've noticed a similar discrepancy between Flyrex and P2V's app. Apps said 4049' and Flytrex max was only 3853', about 5% less. I've not done much high altitude flying but I've never seen the error go the other way.

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I had some extreme altitude discrepancies with FlyTrex when I first implemented it. Once I cut off the foam above the baro, the metrics were very much closer to reality, but not spot on. I'm thinking that the altitude metric as calculated by both GPS and baro aren't necessarily accurate. By the same token, when the P2V hits the ground on an RTH it seems to always lightly touch down. So.... :?:

The autoland algorithm doesn't reference height once it's started to descend. This came up a while back and I had my dealer confirm via his technical contact at DJI that the aircraft will keep coming down gently until it detects it's on the ground, irrespective of height readings. This is what allows it to do a low power autoland, for example, if you run out of juice whilst flying away from home point - you could be over a valley or a higher hill even. It just keeps coming until it hits something that doesn't "give".

I think that the Vision (& presumably the Flytrex) must reset the relative altitude to zero on power on. I have got an expensive watch (present ) that has a barometer and from that it deduces the altitude. I know the barometer is accurate to a fraction of a millibar as i live very close to Liverpool airport and at the same ASL and it it always matches the Metars. If i look randomly at the altitude it can vary +/- several hundred feet. To get a useful reading you have to set it to a known reference point at the current pressure. I think that the fact the app reports take off altitude as being not zero is that from the moment it is set the algorithm is constantly calculating new data.

Staff Member

The autoland algorithm doesn't reference height once it's started to descend. Thus came up a while back and I had my danger confirm via his technical contact at DJI that the aircraft will keep coming down gently until it detects it's on the ground, irrespective of height readings. This is what allows it to do a low power autoland, for example, if you run out of juice whilst flying away from home point - you could be over a valley or a higher hill even. It just keeps coming until it hits something that doesn't "give".